This invention relates to mechanical printers, and more specifically relates to character printing mechanisms of the dot matrix type.
Mechanical printing systems for the data processing industry, particularly those known as line printers, have generally employed formed character images on a member which is moved relative to the paper so as to present a desired type position for an impacting action between the character image and paper. In order to achieve higher speeds, line printers in the recent past have typically employed rotating drums which move vertically with respect to the paper, or a character belt or chain which has horizontal motion with respect to the paper. The character bearing member typically moved in front of the paper, while one or a number of hammers disposed behind the paper abruptly impact the paper against the character member at the proper time. Such printers are the most widely used computer and data system output printing devices, giving print rates of approximately 300 lines per minute and greater. With the virtually constant reduction in the electronic part of system costs over a period of time, however, such printers have become disproportionate in cost, particularly for many lower cost main frame and minicomputer applications.
In addition, the moving character types of systems require extensive maintenance or precise and costly fabrication, to maintain accurate character registration and to minimize image smearing in the direction of character motion. Inherently, such systems cannot accommodate large character sets or variable type fonts, at least without extensive component replacement. They further impose certain limitations on print quality because it is not economically feasible to vary the hammer force, with the result that the intensity of the printed character tends to vary with the area of the raised surface.
More recently, wire matrix printers have been introduced for use with data processing systems, to operate at speeds typically in the range of 50 to 100 lines per minute, and in some instances up to 200 lines per minute. In many of these wire matrix printers, a printer head is used that has a number of separately actuable print wires, one for each possible vertical position within the matrix. The printer matrix head is moved across the front of the paper on a carriage, forming successive characters in a line by impacting against a ribbon which bears against the paper in matrix configurations which define different characters. This technique has substantially reduced costs, particularly for lower speed applications, while permitting a substantial increase in the number of characters in a character set. However, such systems have performance and reliability limitations when operated at high rates for substantial periods of time because of the high rate of usage of the individual printing elements. In addition, such systems have speed limitations, and typically cannot operate at approximately 300 lines per minute or greater. Furthermore, the dot matrix pattern is predetermined by the print head that is used, so that the number and relative disposition of the vertical dot matrix positions cannot readily be changed.
In an attempt to overcome some of these limitations of the dot matrix printers, a movable hammer bank has been devised for a line printer as evidenced by U.S. Pat. No. 3,782,278. In this system, a flexible sheet of hammers, one for each character position, is disposed along a line, and then horizontally stepped across the width of one character with each hammer forming the dots for one character position on that horizontal pass. The paper is then incremented vertically one dot row or line to allow printing of the dots for the next horizontal pass, continuing until the entire character is printed. This system enables line printing with greater speed and without substantial increase in cost, but has a number of disadvantages. To actuate the hammers, stationary hammer actuating mechanisms are disposed adjacent the hammer elements, which are normally in a neutral position and must have adequate clearance. The hammer actuating mechanisms are magnetic, and the clearances needed between the pole pieces of the actuating mechanisms and the hammer introduce substantial air gaps in the flux path, and therefore substantially lower efficiency. The system has certain speed limitations, inasmuch as the movable hammer mechanisms must be incremented laterally to a new position, retracted from the neutral position, fired to imprint, then allowed to settle or dampen at the neutral position before recycling can begin. The incrementing motion of the hammer system relative to the fixed actuators both predetermines and limits the number of matrix patterns that may be imprinted.
There is therefore a general need for a dot matrix system of higher speed but lower cost than has heretofore been available, particularly for line printers. Such a system preferably should have capability for virtually arbitrary selection of dot matrix configurations, type fonts, character sets, and nature of the imprinted data, whether typewriter quality characters, Katakana (simplified Japanese), upper and lower case characters or graphical information are imprinted.